Field of the Invention
The invention relates to a turbine blade which can be subjected to a hot gas flow, including a substrate having at least one interior space and a plurality of bores leading from the interior space out of the substrate, and a heat-insulating-layer system at least partly covering the substrate at a suction side and/or a pressure side.
A product having a heat-insulating-layer system is disclosed in U.S. Pat. No. 4,320,310 or U.S. Pat. No. 4,320,311.
International Publication No. WO 96/12049 A1 discloses the structure of such a heat-insulating-layer system. In that device, the heat-insulating-layer system is formed of a ceramic heat-insulating layer and an adhesive layer. The substrate is formed of a superalloy, the adhesive layer is an alloy of the type MCrAlY containing a portion of the element rhenium as an essential feature, and the heat-insulating layer is formed of stabilized or partly stabilized zirconium oxide. Such zirconium oxide is a mixture of zirconium oxide in the actual sense and at least one further component, in particular yttrium oxide, calcium oxide, magnesium oxide, cerium oxide or ytterbium oxide. The presence of the further component serves to thermally stabilize the zirconium oxide and prevent it from undergoing a phase transformation at the temperatures to be expected during operation. Zirconium oxide is often used as a basis for a ceramic heat-insulating layer, since it has certain mechanical properties which are similar to the mechanical properties of the metals used for the substrate and a possible adhesive layer. Dangerous mechanical stresses between the heat-insulating layer and the metals are thereby avoided at the temperatures to be expected during operation.
European Patent EP 0 486 489 B1 as well as U.S. Pat. Nos. 5,154,885, 5,268,238 and 5,273,712 disclose alloys of the type MCrAlY, which are resistant to corrosion and oxidation at high temperatures and are readily suitable as adhesive layers for ceramic heat-insulating layers.
German Published, Non-Prosecuted Patent Application DE 38 21 005 A1 describes a metal/ceramic composite blade for turbo-machines, in particular gas turbine power units. The composite blade has at least one bulk ceramic part on leading and/or trailing edges which is anchored to a refractory metallic base element of the blade in such a way as to compensate for expansion and in such way that it can be replaced. The blade has a cooling channel inside it, through which coolant can be fed to the pressure and suction side of the blade. There are also cooling-air bores which branch off from the cooling channel, open onto the bulk ceramic part at the leading edge and are closed off by that part. If the ceramic part fractures, the cooling air bores will be exposed in corresponding places, so that it is possible for a secure hot-gas shield to be formed at those points where ceramic elements have broken. Furthermore, German Published, Non-Prosecuted Patent Application DE 38 21 005 A1 gives the option of applying metal oxide thermal barrier layers to the pressure and/or suction outer surfaces of the blade, but without going into detail about the geometrical configuration of the thermal barrier layers.
U.K. Patent Application GB 2 259 118 A, corresponding to U.S. Pat. No. 5,269,653, relates to a gas turbine blade which has an inner cooling channel and is completely provided with a thermal barrier coating. The cooling channel is connected to a cooling chamber assigned to the upstream edge of the turbine blade. Following erosion of the thermal barrier layer and of the base material of the turbine blade in the upstream edge region, the cooling chamber is opened so as to produce laminar cooling of the upstream edge in order to reduce further wearing-down of the base material.
The invention relates in particular to a turbine blade which is constructed as a gas-turbine blade and which is subjected, within the limits of its normal operation, to a hot gas flow that is developed by a flue gas formed by burning a fuel with excess air and has a temperature which can be 1200.degree. C. to 1400.degree. C. on average. Even higher temperatures are taken into consideration and, in order to cope with the problems associated with those temperatures, the development of corresponding gas-turbine blades is steadily advanced. In that case, gas-turbine blades having heat-insulating-layer systems of the type described are considered to be especially important.
A particular problem of a heat-insulating-layer system having a ceramic heat-insulating layer is the brittleness of the ceramic. The possibility of cracks occurring in the heat-insulating-layer system and of the ceramic chipping in the course of normal operation can never be completely ruled out. In that case, the metallic base of the ceramic will possibly be exposed and subjected to the hot gas flow. Any metallic adhesive layer which is present will certainly ensure a certain degree of protection against oxidation and corrosion, especially when the adhesive layer is formed of an MCrAlY alloy or an aluminide. However, due to the loss of the thermal insulation, the adhesive layer will be subjected to extreme thermal loading, so that immediate failure of the adhesive layer has to be expected. That leads to a situation in which the potential of a heat-insulating-layer system with regard to its protective effect will only be utilized with caution, that is it will be less than fully utilized as a rule, within the limits of conventional practice.